Resistivity apparatus for obtaining indications of permeable formations traversed byboreholes



BOREHOLES 5 Sheets-Sheet 1 Feb. 16, 1954 HENRI-GEORGES DOLL RESISTIVITY APPARATUS FOR OBTAINING INDICATIONS OF PERMEABLE FORMATIONS TRAVERSED BY Original Filed Oct. 18, 1949 INVENTOR.

H E N Rl-GEORGES DOLL Hi5 ,nrozemrs.

Feb. 16, 1954 HENRI-GEORGES DOLL RESISTIVITY APPARATUS FOR OBTAINING INDICATIONS OF PERMEABLE FORMATIONS TRAVERSED BY BOREHOLEIS Original Filed Oct. 18, 1949 FIGJB.

5 Sheets-Sheet 2 INVENTOR. HENRI-GEORGES- DOLL HISATTORNEYS.

Feb. 16, 1 HENRI-GEORGES 001.1. 2,669,688

RESISTIVITY APPARATUS FOR OBTAINING INDICATIONS OF PERMEABLE FORMATIONS TRAVERSED BY BOREIHOLES Original Filed Oct. 18, 1949 5 Sheets-Sheet 3 ELECTRlC' ENERGY 4 3 SOURCE OF FIG].

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RESlSTlVlTY- INVENTOR. H EN FRI-GEORGES DOLL BY FIG.2.

Fun-I Z Ikmuo 1954 HENRI-GEORGES DOLL ,669,6

RF'SISTIVITY APPARATUS FOR OBTAINING INDICATIONS OF PERMEABLE FORMATIONS TRAVERSED BY BOREHOLES Original Filed Oct. 18, 1949 5 Sheets-Sheet 4 FIG.3.

INVENTOR. HENRI-GEORGES DOLL 1 wm H/S n'rqkusrs.

Feb. 16, 1954 E RESISTIVITY APPARATUS F0 NRl-GEORG ES DOLL R OBTAINING INDICATIONS TRAVER ED BY S BOREHOLES A 5 Sheets-Sheet 5 F l G. 2 A.

' OF PERMEABLE FORMATIONS Original Filed Oct. 18, 1949 H/S JTI'OR NEYS.

RESISTNITY-v 0 6 INVE NTOR. HENRI-GEORGES DOLL Patented Feb. 16, 1954 RESISTIVITY APPARATUS FOR OBTAINING INDICATIONS OF PEBMEABLE FORMA- TIONS TRAVERSED BY BOREHOLES Henri-Georges Doll, Ridgefield, Conn., assignor to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Delaware Original application October 18, 1949, Serial No. 122,102. Divided and this application May 29, 1953, Serial No. 358,426

11 Claims. 1

This invention relates to apparatus for investigating subterranean earth formations. More specifically, it has to do with novel and highly effective well logging apparatus which is of special utility for distinguishing permeable and impervious formations traversed by a well containing a drilling mud or other fluid having finely divided solids in suspension.

This application is a division of my copending application Serial No; 122,102, filed October 18, 1949, for Resistivity Method and Apparatus for Obtaining Indications of Permeable Formations Traversed by a Bore Hole.

In the present practice, it is customary to use drilling muds in drilling operations in wells. Generally, water base muds comprising finely divided clay or other particles suspended in water are employed for this purpose. Normally, the fluid pressure in the permeable formations tra versed by a bore hole is less than the hydrostatic pressure of the column of mud in the hole, so that the mud and mud filtrate flows into those formations. However, such formations tend to screen out the finely divided particles in the mud, so that a substantial mud cake is formed on the wall of the bore hole at the levels of the permeable formations. On the other hand, the mud does not flow into the impervious formations so that substantially no mud cake is formed on the wall of the bore hole at the levels where they occur. The presence or absence of a filtration mud cake on the wall of the bore hole, therefore, affords a reliable indication of whether the formations at different depths are permeable or impervious.

It is an object of the invention, accordingly, to provide novel apparatus for determining the presence or absence of mud cake on the wall of a bore hole atdifferent depths therein.

Another object of the invention is to provide novel apparatus in which electrical resistivity measurements are utilized to determine whether or not a mud cake has been formed on the wall of a bore hole.

Yet another object of the invention is to provide novel apparatus for distinguishing permeable and impervious formations traversed by a bore hole by obtaining indications of the electrical resistivity of material contiguous to the wall of the bore hole.

A further object of the invention is to provide novel apparatus of the above character which is capable of indicating at the surface of the earth the approximate thickness of any mud cake on the wall of a bore hole.

Still another object of the invention is to provide novel apparatus for obtaining simultaneous indications of the diameter of the bore hole and of the presence or absence of a mud cake on the wall of the bore hole whereby more reliable mud cake determinations may be made.

According to the invention, highly localized measurements are made, in situ, of the electrical resistivity of the materials comprising the wall of the bore hole. More particularly, at least two wall resistivity measurements are made at each location, one of which is influenced, to a marked degree, by the material lying in a narrow zone extending laterally from the wall of the bore hole a short distance into the surrounding material.

The other resistivity measurement is influenced to a lesser degree by material lying in said narrow zone and to a greater degree by material located beyond said zone.

By making the width of said narrow zone of the same order of magnitude as the anticipated thickness of the mud cake, the first of said resistivity measurements will be influenced by the mud cake to a greater extent than the other. Since the resistivity of the mud cake is usually different from, and relatively low to, the resistivity of other formations surrounding the bore hole, the two resistivity measurements will be relatively low in value and will differ, when a mud cake is present. In regions where no mud cake has been formed, however, they will tend to be substantially the same.

pervious formations traversed by a well.

The invention may be better understood from;

the following detailed description of several typical embodiments thereof, taken in conjunction with the accompanying drawings, in which:

'Fig. 1 is a schematic diagram, partially in longitudinal section, showing typical well logging apparatus constructed according to the invention, in position in a bore hole drilled into the earth;

Fig. 1A is a view in elevation, partially in section, showing the electrode assembly of Fig. 1 in' locking the bowed springs to the body and subsequently releasing them;

Fig. 2 is a typical log such as might be obtained It will be apparent, therefore, that the invention enables permeableformations to be readily distinguished from im- U with the well logging apparatus of Fig. 1 in a bore hole;

Fig. 2A is another example of a log illustrating the type results obtained when disturbing efiects are present opposite impervious formations;

Fig. 3 illustrates schematically another form of the invention utilizing a difierent electrode as sembly and another form of locking means;

Fig. 3A is an end view of the apparatus shown in Fig. 3 with part of the electrode .supportbroken away to show the interior construction thereof;

Fig. 3B is a detailed view of the rein-forcing spring for the electrode support;

Fig. 3C is a view, partially in iongitudinaL-section, illustrating another form of locking and releasing means which is used with the apparatus of Fig. 3;

Fig. 4 is a schematic diagram of another modification shown with the electrode assembly mounted in position in a bore hole;

.Figs. .5 and 6 are views in elevation'of modified forms of electrodes;

Figs. 5A and 6A are enlarged views in section taken .along the lines .5A-J5A and liA--fiA-of.Figs. 5 and B respectively, and

Fig. '7 illustrates schematically .another modification in which the resistivity measurements are .made simultaneously with a calipering oooration.

In the form of the -.apparatus shown in 1, the electrode assembly comprises a tubular support in which is adapted .to be lowered into a bore hole 1-] on a supporting cable 42 which may be raised and lowered in the well .by suitable means (not shown) located at the surface of the earth. The bore hole ll usually contains :a column of morev .or less conducting liquid 1 Mounted on the tubular support L9 is a spring cage assembly [3 comprising a plurality of bowed springs I and L5 whose opposite ends may sbe rigidly secured to a pair of collars It and J 1, respectively, which are :slida-bly mounted on the tubular member Id. The springs I4 and 4.5 are so shaped that. the intermediate portions thereof are continually urged towards the wall of the borehole. Stop means 18 and is may :be formed on the tubular member iii to .provide for limited longitudinal movement of, the spring cage as-- sembly .13 with respect to the tubular member .Hi.

The bowed spring M carries a wall engaging cushion member made of suitable flexible insulating material such as rubber, for example. Formed in the side wall of the cushion 20 are a plurality of recesses 25, 22 and 23 within which are embedded a plurality of electrodes A, M and. M, respectively. Preferably, the electrodes A, M and M lie beneath the surface of the. crashion 20 so that they are spaced a short distance away from the wall of the. borehole when theapparatus is disposed in the well, as shown in Fig. 1,, electrical communication between the several electrodes and the adjacent formation being efiected through here hole liquid. entrapped in the recesses 72!, 22 and 23. With this construction, rubbing of the electrodes against the wall of the bore hole is prevented so that spurious electric potentials which might be created bythe rubbing action areavoided.

In order to maintain the electrodes A, M and M. in fixed, closely spaced relationship with respect to the Wall of the bore hole U, the bowed spring i is provided with an intermediate straightv portion 2% to which is secured a straight, rigid reenforcing member 25. By virtue of this construction, the electrodes A, M and vM ar aldirection, as shown .in Fig. 1B. struoticn, the cushion 2t will exert sufficient pressure against the wait of the hole, for all bore hole the relay winding to a conductor diameters lying in a given range, to squeeze out the borehole .fiuid from between the cushion 28 .and the wall, so that the shunting effect of any remaining film of fluid on the resistivity measurementswillbe negligible.

In order to facilitate lowering the apparatus in hose .holes of small diameter, it is desirable to provide means .for locking one of the spring cage collars, say the collar 1:7, to the tubularmember it at a position far enough away from the other collar is so that the cushions 253 and 2% will be out oi engagement with the Wall of the borehole ii. A. typical locking device suitable for this purpose is shown in Fig. 1C and it comprises a.

metal plug 25 threadedly or otherwise secured at the bottom of the tubular member is andhaving a powder chamber 2! formed therein ccmmun'i eating with an opening within which a blunt projectile 2:? is adaptecbto be received.

While the apparatus is at the surface of the earth, the lower collar H is pulled downwardly until the cushions Z8 and 2 6. have been moved inwardly the desired amount. The collar l'l may then be lock-ed in this position by means of a metal strap which may be secured to the col lar i 2 by screws 3i, for example. The strap passes around the plug 26 and is held tightly in a slot 36 therein so as to retain the blunt projectile 29 within the chamber 28 as shown in Fig. 1C.

Ignition of the charge in the chamber .2! may, be accomplished in any desired manner, as, for.

example, by means of an electrical igniter 372 grounded to the plug 26 at and having its other end connected to-a. conductor 34. The conductor 34 passes through suitable insulating ma terial 35 in the plug 25 to one contact 37 of a Tu" lay-38.

The relay 3 3 may be of the type havin .a movable contact 33 normally engaging the fixed con-.

tact 3i and which is connected in series with The conductor its extends through the supporting cable 12 (Fig. l) to the surface of the earth where it is connected in series with a suitable current indicating instrument H, a variable resistance 42, a switch -52, a source of electrical energy 43 and a conductor to a ground electrode B which may be formed on the supporting cable 12a relatively great distance from the electrode A.

The electrode A is connected by a conductor 45 (Figs. 1 and 11C) to another contact 45 on the relay 35. The electrodes M and M (which are potential measuring electrodes) are connected together by the-conductors and 38, respectively, through a conventional high impedance potential recording instrument located at the surface of the earth. The electrode M is also connected through the conductor d? to a second high impedance potential recording instrument 5,9, the other terminal or" which is grounded at the point 5i at the surface or the earth.

It will be understood that the electrodes A, M and M constitute essentially a three electrode resistivity system of the type shown in prior Patent No. 1,819,923 to Conrad Schlumberger. lhus, the log recorded by the instrument 49 is representative of the electrical resistivity of the earth as measured by a three electrode system having an electrode spacing equal to the distance between the electrode A and the midpoint between the electrodes M and M under the cushion 242. Since the electrodes are substantially insulated from the mud column of the bore hole, substanv tially hemispherical rather than "spherical" resistivity measurements are obtained. Further, the electrode spacing is made very small, i. e., from one to a few times the mud cake thickness so that the log made by the recording instrument 49 will reflect variations in the apparent resistivity resulting from any mud cake that may. exist on the wall of the hole.

opposite permeable formations is usually of the magnitude of a fraction of an inch. The following electrode spacings have been found satisfactory in the field: AM=1 inch, AM=2 inchesr The electrodes A and M constitute a two electrode system of the type disclosed in prior Pat ent No. 1,894,328 to Conrad Schlumberger and the log provided by the recording instrument 53 is representative of the electrical resistivity of the formations as measured by a two electrode system having an electrode spacing equal to the distance between the electrodes A and M under the cushion 20. It is well known that a three electrode system has a shorter depth of lateral investigation than a two electrode system having the same electrode spacing. For the spacing of two inches suggested above, therefore, the log produced by the two electrode system will reflect the resistivity of any mud cake that may be formed to a lesser extent than the log obtained by the recorder 49, and it will be influenced more by the resistivity of the formation behind the mud cake.

In operation, the spring cage assembly 13 is lowered into the bore hole I i in the locked position, as shown in Fig. 1A. When the desired level has been reached for the logging operation to begin, the switch 52 at the surface of the earth is closed so that electrical energy from the source 43 is impressed upon the electrical igniter 32 (Fig. 1C). This explodes the charge in the chamber 21, ejecting the projectile 29 and breaking the metal strap 33. The collar I 7 is then free to move upwardly until the cushion members 26 and 2E? are in engagement with the Wall of the bore hole H, as shown in Fig. 1. The projectile 29 should preferably be made of soft material which will not interfere with subsequen drilling operations.

Simultaneously, the interruption of the cur rent through the relay 38 causes its movable contact 39 to disengage the contact 3? and thereafter to remain continuously in engagement with the contact 45 so that electrical energy is impressed upon the electrodes A and B and current is emitted from the electrode A into the surrounding formations. The flow of current into the formations from the electrode A produces a potential difference between the electrodes M and hole I i where a mud cake may be expected to be present opposite permeable formation. On the other hand, the potential difference between the Experience hasindicated that the thickness of the mud cake 6 electrode M and the ground 51 at the surface of the earth is a function of the resistivity of the material at a greater lateral distance from the electrode A. That potential difference, therefore, is less influenced by any mud cake that may be present on the wall of the bore hole.

Experience shows that the mud cake which forms opposite permeable formations usually has a lower resistivity than that of a zone of lateral extent in the formation behind the mud cake which has been affected by the filtrate from the drilling mud, and a slightly higher resistivity than the mud itself. Good contrast in resistivities between the mud cake and the formation ocours in those geological strata containing hard formations such as limestone. The important feature in each case is that the resistivity of the mud cake be different from the material behind it. By means of such resistivity contrasts, the presence or absence of a mud cake, therefore, can readily be determined by comparing the logs obtained by the recording instruments 49 and 50, and thereby the presence or absence of permeable strata is indicated.

If the sensitivities of the recording devices 49 and 50 are adjusted so that the logs which they produce have the same amplitude when the resistivities measured in the hole are the same, and are superimposed, a composite log of the type shown in Fig. 2 will be obtained in a typical well in the field. In Fig. 2, the two logs are identi-'- cal in the range from 7,050-7,100 feet. The logs are also identical between 7,130 and 7,200 feet and between 7,225 and 7,250 feet. The identity of the two logs in these regions indicates that the two resistivity measurements are the same and that a negligible amount of mud cake is present. Hence, the logs provide clear indications that the formations at these levels are impervious.

On the other hand, between 7,100 and 7,130

feet and between 7,200 and 7,225 feet, the log L49 (made with the recording device 49) indicates a lower resistivity than the log L56 (made with the recording device 50). Since the log L49 is a function of the resistivity of the formationsin a zone very near the electrode A, while the log L50 is a function of the resistivity of the for-' mations located a greater distance from the electrode A, the discrepancy between the two logs L49 and L50 provides a clear indication that the formations between these levels are covered by a mud cake and are, therefore, permeable.

In practice, it has been found that the curves sometimes become separated opposite impervious formations, but the order of the apparent resistivity values is reversed compared to the curves obtained opposite permeable formations. Such a situation is illustrated in Fig. 2A. The permeable formations are identifiable, however, as in Fig. 2, by the low values of the resistivity indications for the shorter spacing, given by curve L69, compared to the longer spacing curve L50.

In Fig. 2A, a thick permeable zone is clearly indicated between the depths of 8583 to 8598 by the difference in, and the character of, the resistivity indications of the curves L49 and L50. In the section below 8619', the curve L49 of the shorter electrode spacing is higher in value compared to the longer electrode spacing curve L50, and this section is known to be impervious. ence of permeable streaks where the curve L49v has minima below the curve L59.

The situation over the impervious section could be due to the use of a cushion of limited areav The other parts of the log show the pres aeeaoss with: to. the spacings between; the-else:- trodes; conjunction. the: presences of. a film mud remaining between: the. cushion: and the: formation. would. provide. a shunting eifecizwhiclzr would afie'ct the apparent resistivity indication. for? the longer: spacing: to a greater degree. as compared to the shorter spacing; but. nevertheless. a discrimination: can" be made vbetweennpermeable and impervious formations. Ii deemed: necessary; though, experience shows; thatthe; type. log; of. Fig. .2; can beobta-ine'd by; career in the: construction and use. Off the; ap paratus;

There: is: another: noticeable: distinction}. which f5? oi; diagnostic, use; determining the perms-l ableeformations. As:illus.trated, in.Figs...2:and: 2A tiiezresistivity indicationsobtained? opposite: per-'- meable. formationsz'have: anappreciably smoother. contour: than: those. resistivity indications. oppoimperviousformations: due; to the: presenceofi'a substantial. mud: cake, which of moreunitome composition. than the geological forma. ti'om In the embodiment shown in.Fig-.13,.twc;simultans-cue resistivity measurements. are obtained by. two: pairs-oi potential electrodes; M, N-,;M.f, Nf, the'electrodles M, N'being connected to theapoten recording: instrument. 49 through: the. cable conductors We. and. 413,.and the electrodes N being; connected to.- the potentiali recording. instrumeniriil through the conductors. 5.3 andi 54 Three. electrode? systems". arev preferable; to: two electrode: systemsfor investigating; thin: mud: cakes-2. In. a. typicals, case; the: electrode. spacings A.-M-,. M.--N, (Fig. 3):- mayrb'e of. the'magniinlde ofi oneequarter: of an. inch; for example-y the. spacing: between the. electrodes Ne-M': being" about one-half: an inch, .for exam. ple.

Rig. 3* also.- illustrates... a. modified: spm'ng cage construction ELSJWGH aszan al'ternateafhrmoi'locksing; device: that may be"; employed: accord-.. ance; withthe. invention; shown}. the spring. cage: 55 comprises a: pair of bowed: springs 56- and; 51" which. are: pivcttally mounted;- at. their: upper: ends-5B and 531m: the collairlfii llhe cote 1811" is: rigidly. secured. to: the tubular member Hi2.

by screws: 'lfiiion example; The lower GIIdSJ off the;springs 56 and 55: are: pi'votally mounted-1 at Ell; and-16F ornthezlowercollar. lL. The-:bowedz spring." 552 is provided-a with: an inwardly depressed? cesses: 6'4, 55', 66. ET" and 58 formed in arcushions 69 made-1 of" flexible: mediating material suclr as rubber; for example, such. that: a nan.

row space exists: between: eachelectrode and: the? wall: of "thee-bore hole when in the position. shown: iniFig; 3;

The bowed spring' 511 may also" be provided with a cushion member'll to facilitate movement. of the apparatus in: a well.

Embedded in the cushion member68 is apl'wr ality of' strong flexible springs: T0; TI, 12.; 13, 114 and 15 which may be secured at: their oppoisite ends it to: the bowed spring 56', in any suit-- able manner; as by-wel'ding, for exampler These springs'z'tend" to cause the cushion members to" straighten outalong a transverse-line therein, so that it will remain in engagement with the wall ofthe bore hole even though the bore hol'e diameter varies, as wherethe" apparatus is" to be used in bore holes of difierent diameter, for example:

Imordertakeep the: cushion: members: 68 and 1:1? out of engagement: with. the. wall of? the: bore hole as the apparatus. is. lowered. therein; the. lower collar If! istadapted. to: be pulled down wardly and 'lockeditoi the. tubular member in by means ofi the locking device shown in Fig. 3C. Fig; 3C, thelowercollar I is provided with: in' ternal? teeth 18i which are adapted to slidain-a: longitudinalslotlW in the member l0 andwhich are adapted to be engagedv by a pawl 19 -pivotially mounted 'at-BU within the tubular: member H15. Secured on the pawl I9 is a. conventi'onal spring 8| which serves to keep the: pawl 18 nor mail? out of engagement with the internal teeth- 18 Alsopivoted at 89 is a cam arm. 82' engaging acam' member 83 mounted on a rod 31'! which is adapted to be actuated by asolenoid. 85 secured withiniihetu'bularmember It. Mounted onthe' cam arm 82 is: a conventional spring 86 which engagessthe' pawl T9 to cause the latter tomove intoengagement with" the internal teeth m.

Normally, the cam member 33 ismaintainedl in its uppermost position shown in dott lines in Fig. 3G by means compression spring 8' 3; The solenoid 85 may be energized from the surface of" the earth in: any suitable manner. For example, one of its terminals may be grounded. to the tubular. member it at 88 and its other terminalmay'be connected by a' conductor SE to onefixed 7? contact 9% of a switch 9F having a movablecon iii) tact 92 (Fig. 3 The movable contact 95. may be connected in series with the currentmeasuringinstrument 4-! and the other fixed contact may be connected to the conductor it" which leads-to theel'ectrode A in the'bore hole H. With this construction, it will be apparent that the source of electrical energy can be connected either to the solenoid 3501' to the electrode-A by manipulating the switch 9!.

Beforethe apparatus shown in Fig. 3 is lowered into the'well, the lower collar I7 is pulled downwardly a sufficient distance'to insure that'- thecushions G9 and I? will not engage the wall of" the bore hole and the switch contact $2 is moved into engagement with the contact 98' (Fig. 3). Thi's'energizes the'solenoid 85, moving the cam member 83 downwardly and causing the arm 82 t'obe moved towards the right to the positionshown in full lines in Fig. 3C. This causes-the pawl= 79- to engage the internal teeth it, thus locking the lower collar IT to the tubular member- Ill;

The-spring cage assembly 55 is then lowered into the well, and, when the desired depth has been reached. the movable switch contact 9-2 (Fig: 3) is disengaged from the fixed contact 36* and 'moved into engagement with the contact 93.

This deenergizes the solenoid 35, releasing the pawl 79 so that the collar I? can move-upwardlyuntil the cushions 5e and Ti! are in engagement with the wall of the bore hole. At the same time; the source of electrical energy 4.3- is con-- nected to the electrode A so that resistivity logs maybe obtained as the apparatus is raised in the well.

One'ad'vant'age-of the locking device shown in Fig. 3C is that it enables the collar i? to be locked to the tubular member Hi after only a portion of thebore hole has been logged. Thus, after completion of the desired portion of the log, the switch 91 (Fig. 3) is manipulated to-bring the movable contact 92' into engagement with the contact 90, energizing the-solenoid 535' and causingf'tlie pawl Hi to engage theinternal teeth l8 (Fig. 31?); As the-spring assembly 55 raised in the bore hole, the collar II will be locked in a position corresponding to the smallest bore hole diameter encountered since it can slide downwardly even though the pawl 19 is in the locking position. Hence, the device can be withdrawn from the hole at a greater speed with less wear on the cushions 69 and TI.

' In the embodiments shown in Figs. 1 and 3, there is a remote possibility that the insulation between the conductors in the cable may be decreased such that a portion of the current intended for the electrode A may leak into the resistivity measuring circuits, thus giving erroneous indications. To eliminate this possibility, the current source may operate at one frequency and the potentials received by the potential electrodes may be converted to different frequencies before transmission to the indicating apparatus at the surface, as shown in Fig. 4.

In Fig. 4, closely spaced apart current emitting electrodes A and B embedded in the cushion 59 are connected by the conductors H6 and II? to the secondary winding 94 of a transformer 95, the

primary winding 96 of which is connected through the conductors 9'! and 98 in the cable I2 to a source of alternating current 99 of suitable frequency, say 400 cycles per second, located at the surface of the earth.

The potential difference between the electrode M and a reference electrode N mounted on the cable a considerable distance therefrom is impressed upon the input terminals of a conventional amplifier rectifier I09 which amplifies the A. C. potential picked up and converts it to direct current. The direct current output of the am-- plifier Hill is transmitted through the conductors I90 and I02 in the cable I2 to a suitable recording instrument I03 at the surface of the earth, a-

conventional low pass filter I04 being interposed,

- if desired, to eliminate A. C.

Similarly, the A. 0. potential picked up between the electrode M and the reference electrode N is impressed upon the input terminals of another conventional amplifier rectifier I85, the D. C. output of which is transmitted through the conductors I66 and III! to the conventional D. C. recording instrument I08 at the surface of the earth. If desired or necessary, a conventional low pass filter I09 may be interposed between the instrument I08 and the conductors I35 and Ill! to keep A. C. out of the instrument Hit. The manner of operation of this embodiment will be readily apparent from the several other forms of the invention that have been described above.

It will be understood that the amplifiers I00 and I05 should have substantially the same temperature characteristics. Obviously, amplifier modulators may be employed instead of the am-' hole caliper of the type described in application I Serial No. 785,270, for Mutual Inductance Systems, filed November 12, 1947, by Owen H. Huston.

In Fig. 7, the lower ends of the bowed springs I4 and I5 are secured to a connecting member I I 0 10 I which carries a rod-like member III made of suitable magnetic material. The rod-like member III extends into an opening H 3 in the bottom of the tubular member II) and is adapted to modify the coupling between two inductively coupled windings (not shown), one of which is energized by a source of alternating current II2 located at the surface of the earth and the other of which is connected to a suitable recording device II3, also located at the surface of the each. The log made by the recording device I I3, therefore, is a function of the diameter of the bore hole I I.

Preferably, the two resistivity measurements recorded by the devices 49 and 50 and the bore hole diameter measurements indicated by the recording device II3 are recorded on the same log. It will be apparent, therefore, that if the cushion 28 were passing an enlargement or cave in the bore hole of such diameter that the cushion would not be pressed against the wall of the hole, the log would indicate this fact. It would then be known that the resistivity indications derived from potential measurements made with the electrodes M and M at that level will be altered by the resistivity of the drilling mud. This combination is highly desirable since it enables the resistivity logs to be accurately correlated with the caliper log, an object which is very difficult to attain where the resistivity logs and the caliper log are made in separate runs in the well.

In certain cases, the mud used in the bore hole does not form a thick mud cake but is designed to form a thin protective coating over permeable formations to prevent the entry of bore hole fluid thereinto. In order to obtain indications of very thin mud cakes of this type, in accordance with the invention, the spacings between the several electrodes must be made exceedingly small. In such case, if small circular electrodes were employed, as shown in Figs. 1, 3 and 4, their resistance might be so high as to affect the measurements adversely. In such applications, therefore.

, electrodes of the type shown in Figs. 5, 5A, 6, and

6A may be employed.

In Fig. 5, three thin blade-like electrodes A, M and M are embedded in the flexible cushion 20. As indicated in greater detail in Fig. 5A, the

1 three electrodes are preferably disposed below the wall of the cushion 2!) so that they will not rub against the wall of the hole while the measurements are being made.

It is also possible to achieve the same result 1 with circular electrodes A, M, M made of thin material such as wire, for example, and embedded in recesses in the cushion 20, as shown in greater detail in Fig. 6A.

It will be understood, from the foregoing description, that the invention provides novel and highly effective apparatus for determining whether or not a mud cake is present on the formations located at different depths in a well. By making extremely localized resistivity measurements at two lateral depths of investigation which are suitably selected to provide measurements that are influenced to different degrees by any mud cake that may be present, the presence or absence of mud cake may be readily accertained, so that permeable and impervious formations may be readily distinguished. In fact, the simultaneous resistivity values obtained for the different spacings can be arranged to be compared automatically, if desired, rather than by a visual examination of the records,

It will be understood that the auxiliary cushions 20' in Figs. ,1 and 7 and 11 in Figs. 3 12116.4 can also be adapted to carry other electrode arrangements such as, for example, an electrode pressed against the wall .of the bore hole for measuring spontaneous potentials.

Where reference is made in .any of .theiollowingplaims to the conversion-of apotential diifference of one frequency ,to a signal of diifcre1 1;t;freq ency, this is intended to incl ude both the .conversion of direct current to alternating current and the conversion .of alternating current to direct current or to alternating current of different frequency.

According .to conventional ilsage, has been followed in the foregoing description, in systems including at least four electrodes (two current andtwopotential) in which indications representative essentially of potential gradient are obtained (e. g., by means includinga pair of nearby electrodes such .as the potential electrodes M and M in :Fig. 1, .or the current electrodes A and B in the equivalentreciprocal system shown in Fig. e), the electrode spacing is-vdefined as the distance from the midpoint between the two nearby electrodes ,of the same category (current or potential) to the nearest electrode of differentcategory. .On ,theother hand, where indicationspfthe absolute potentialtof an electrode are obtained (e. g .thezelectrode .M' inFfig. 1 the electrode spacing is the distance from that-:electrode to the nearest electrode of difierentcategory. Where reference is made in the following claims to the electrode spacing in an electrical logging system, the conventional meaning of this rm nolo y i i l din the embodiment shown in Figs. 1A, 1B and 16 any other suitable rnechanism may he employed instead of the relay 35 for firing the igniter .32. ,Eor :gexarnple, a conventional clash- -Qi im delay re a o th type u d n h selective f ring of charges in ;a gun perforator a be emplo ed aio thi turn o o- Alternatively, the conductor .40 might be directly connected to the el ctr de nandthe e to .2 conn ct d to the source of electrical energy .43 by vmeans of a separate circuit ;l,ike that employed .for the solenoid .85 .in Fig. 3C, .for example. in v such caee, no relay switching means would be required.

Obviously, the seve a y i a cm-bo described a ve ar us eptible of no modifications inf rm andidetail Wi n-$ of the invention. For example, any sui H source .Of ele t i al energy ma -bcc pioy i a a .A- C- :o at.cd -D- 0,. f r s mplo provided that appropriate indicating .mea s are employed, .as ,is well known in the art. Further, either form of cage assembly or looking deyice may .be .used eindifierently with any threeof the specific logging ,circuits disclosed. ,Also, any th snitable electricaloircuits may beemployed for ,obtaining the .two resistivity .measurements according to the invention. The; specific embodimeri s disclosed herein, therefore, areanot :to be regarded .as imposing vany restrictions 'whatso ever upon the scope of the dollowing claims.

:Lclaim:

1. In awell logging apparatus, :the combination ciafiexible insulating cushionimember mounted for movement through :a well and having :an out r fa e, Jncans for Fo c sai member deterally to :bring the outer face thereof intoengagement with the wall f the :well, a plurality of small, closely spaced electrodes A mbedded said cushion member iouter face :at .di ffierentdistances from @31 5 on oflisaidwlegirqfligs, 39 cold electrodes havin ,anio posod zP-ontion a ns th wallpf the well, means forpassing electric current between said first electrode and a ltcference point spaced apart therefrom, a fi st poten ial indicating circuit connected to receive the potential difference betwaen a Second .of said electrodes and a reference point spaced apart therefrom, said first and second electrodes corn? p s n p of a fir t osp r nsc t m h ving a very small electrode spacing adapted to render the systemresponsive primarily to the .electnical resistivity of the {material in a;zone,on theorder of .a fraction of .an inch in thickness extending la ra ly nd utwardly o th su fa ho indary of the bore hole, ,a second potential :indicating circuit connect d .to receive the potential difi c etwe n a hi d o sa d ct od s and a reference point spaced apart therefrom, ,said first and third electrodes comprising part of a second explori ystem h vin an el ctrode p n reat an said first exp ori g eastern and rendering said second exploring system priiarily responsive to the electrical resistivity ,of the .earth ,formation material beyond said gene, and means for recording .the potentials indivoted by ,s d fi i an second pot n ind cat ing circuits.

c m nation-in the .well io ginsa parat defined in claim 1, Joore hole ealipering means mountedfor moye mint through the .well in fixed relation to said flexible -,insu1a'ting=;cushion.mem.- her-and adapted to provide an electric signal responsive to variation in .a lateral .dhnensionof the .bore hole.

3. In combination in well logging apparatus as defined in .claim 1, a support adapted to he lowered into the .wel1 ,,at 1e3Sf/;two ,bowed spring elements secured together at opposite endaat least one of said ends boingslida'ole longitudinally withrespectto-said support, said flexible insulating cushion member beingsecured .onone .of said bo cdsp ins ele ents a re e s le-m an ca ried by thc-sup ort o loo ing-said bowed spring el ments o the support in a -.contraotcd p si on- 4. ;In combination well logging apparatus ,as d i d i cla a up r adopt t be lowered into'a welL-and at ,leasttwobowed spring elementssecured together at their opposite ends, at ast o e o .saidkends seine-moun ed .on said support for limited .long'itudinal sliding move.- ot with respe t the et sa d ificxi c nsu tin ushi mem e home s c re on o o Said b wledoc ti scl mcntaand o d n c c s rin element having a su sta tia ly tra ght. inwardly depress d po t on su ta y c o o ocd to ma nain a po o .o .--so d sh n m mbcc o iace ineng'aeemen with the wal o the .wel

.5. Wel logg oo a tu uas d fined .in cla m 1,, in which sa d electrodes c mprise c osel spaced parall l b a o i c e em n embedded said cushion so as to hesnacedapartirom .the a ler. W l close oxim ty the et 6. In ccmhination well logging apparatus as (def ned in claim a u p rt ad pte .to h owcmiinto th WclLl-at leas two .bowod.- .orins lcmc t sc u c to ether at t oop sit at least one o sa d ends be n mount d. for relat ve ov me t .With r spe t o sa suppor inccspQnc' t yariation vi ihor hol diameter. said.fl x bleinsulatin -cu hio .mcmherheing secu od onpoeo -said bowed, priosi omcntsland means responsive to relative movement of said onciood withir s c toihc soppc t f Pro di inoicatiorioiof -variations in bone .hole diameter.

7. In well logging apparatus, the combination of a flexible insulating cushion member mounted for movement through a well and having an outer face, means for urging said member laterally to bring the outer face thereof into engagement with the wall of the well, a plurality of small closely spaced electrodes embedded in said cushion member at different distances from a first one of said electrodes, each of said electrodes having an exposed portion facing the wall of the well, a first electrical circuit having as terminals said first electrode and a reference point spaced apart therefrom, a second electrical circuit having as terminals two other adjacent ones of said electrodes, the spacing between said first electrode and the midpoint between said two other electrodes being not greater than about one and one-half inches, a source of electrical energy in one of said first and second electrical circuits, electrical indicating means in the other of said first and second electrical circuits, a third electrical circuit having as terminals one of said electrodes and a reference point spaced apart therefrom, said last-named electrode cooperating with the one of said first and second electrical circuits containing said source of electrical energy to form an exploring system having an electrode spacing greater than one and one-half inches, and electrical indicating means in said third electrical circuit.

8. Well logging apparatus as defined in claim 7 in which the flexible insulating cushion member is secured on one of two bowed spring elements which are connected together at opposite ends, at least one of said ends being slidable longitudinally with respect to a support adapted to be lowered into a well; the source of electrical energy provides current of one frequency; and both of said indicating means are provided with means for converting potential differences supplied thereto to signals of different frequency.

9. In well logging apparatus, the combination of a flexible insulating cushion member mounted for movement through a well and having an outer face, means for urging said member laterally to bring the outer face thereof into engagement with the wall of the well, a plurality of small, closely spaced electrodes embedded in said cushion member outer face at different distances from a first one of said electrodes, each of said electrodes having an exposed portion facing the wall of the well, means for passing electric current between said first electrode and a relatively remote reference point, means for providing indications of the potential difference between second and third ones of said electrodes, and means for providing indications of the potential differenoe between one of said second and third electrodes and a reference point substantially at electrical infinity, the distance between said first electrode and the midpoint between said second and third electrodes being of the order of one and one-half inches, and the distance between said 14 first electrode and said one electrode being greater than the distance between said first electrode and said midpoint.

10. In well logging apparatus, the combination of a flexible insulating cushion member mounted for movement through a well and having an outer face, means for urging said member laterally to bring the outer face thereof into engagement with the wall of the well, a plurality of small, closely spaced electrodes embedded in said cushion member outer face at different distances from a first one of said electrodes, each of said electrodes having an exposed portion facing the wall of the well means for passing electric current between said first electrode and a relatively remote reference point, means for providing indications of the potential difference between second and third ones of said electrodes, and means for providing indications of the potential difference between adjacent fourth and fifth electrodes, the distances between said first and second, second and third, and fourth and fifth electrodes being of the order of one-quarter inch and the distance between said third and fourth electrodes being of the order of one-half inch.

11. In well logging apparatus, the combination of a flexible insulating cushion member mounted for movement through a well and having an outer face, means for urging said member laterally to bring the outer face thereof into engagement with the wall of the well, a plurality of small, closely spaced electrodes embedded in said cushion member outer face at different distances from a first one of said electrodes, each of said electrodes having an exposed portion facing the wall of the well, means for passing electric current between said first electrode and a second of said electrodes adjacent thereto, a first potential indicating circuit connected to receive the potential difference between a third of said electrodes and a reference point substantially at electrical infinity, and a second potential indicating circuit connected to receive the potential difference between a fourth of said electrodes and a reference point substantially at electrical infinity, the distance between said third electrode and the adjacent one of said first and second electrodes being not greater than about one inch, and the distance between said fourth electrode and said adjacent one of said first and second electrodes being greater than the distance between said adjacent one electrode and said third electrode.

HENRI-GEORGES DOLL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,268,137 Evjen Dec. 30, 1941 2,268,138 Evjen Dec. 30, 1941 2,414,194 Ennis Jan. 14, 1947 2,427,950 Doll Sept. 23, 1947 

